The invention relates to a fuel injection valve for high-pressure injection of fuel from the high-pressure reservoir into a combustion chamber of an internal combustion engine as generically defined by the preamble to claim 1.
One such fuel injection valve is described in German Patent Disclosure DE 196 19 523 A1. In this known injection valve, the control of the injection is effected electrohydraulically, by delivering fuel from the high-pressure reservoir at high pressure to a control chamber. By means of this control pressure, the valve member of the fuel injection valve is then kept in the opening position. This is attained because the control surface area of the valve member acted upon by the control pressure is greater than the area acted upon of the shoulder of the nozzle needle of the fuel injection valve. The control chamber communicates permanently with the high-pressure reservoir, specifically in throttled fashion, and it can be relieved via a further throttle with a throttle conduit segment. This latter throttle is controlled by a magnet valve. As soon as the magnet valve opens this throttle, the control chamber is relieved, and as a result the pressure at the pressure faces of the valve member of the injection valve is sufficient to allow it to be put into the open position or in other words the injection position, during which the injection takes place. If this throttle is now closed again by the magnet valve, an increase of pressure occurs in the control chamber, and as a consequence the valve member is returned to the opening position. The fuel injection valve also has a relief line, leading away from the electromagnet of the magnet valve, and the fuel quantity diverted at the aforementioned throttle can flow out to a relief chamber via this line.
A similar basic layout is also described in European Patent Disclosure EP 0 661 442 A1. The throttle, which is controllable by means of the magnet valve and which seals off the side of the fuel injection valve toward the relief line or opens it and connects it with the control chamber, has two cylindrically embodied conduit portions. The first conduit portion, which represents the actual throttle conduit cross section, is relatively long compared to its diameter and discharges with an abrupt change in cross section into the second cylindrical portion, which has a considerably larger cross section and establishes the communication between the actually thin, long throttle conduit segment and the control chamber.
Especially in the case of the slender, long and thin throttle conduit segments, the flow of fuel has enough time to develop in such a way that the fuel presses against the conduit walls. With such narrow throttle conduit cross sections, however, this entails not inconsiderable flow losses. To reduce the flow losses, the course taken is to lap the conduit walls of these throttle conduit cross sections in order to reduce their roughness. This involves major technological effort, however, which means high production costs. Particularly with very narrow throttle conduit cross sections, this kind of superfine surface machining runs up against the limits of technical feasibility, taking reasonable costs into account.
The object of the invention is therefore to create a fuel injection valve whose throttle between the magnet valve and control chamber can be manufactured at little production effort or expense and with low flow losses.
This object is attained with a fuel injection valve is having the characteristics of claim 1. Expedient refinements are defined by the dependent claims.
The fuel injection valve according to the invention for the high-pressure injection of fuel from a high-pressure reservoir into a combustion chamber of an internal combustion engine, specifically a diesel engine, has a magnet valve by means of which the fuel pressure in a control chamber, via a throttle which has at least one throttle conduit segment with conduit walls, can be relieved, which corresponds to the injection position of the fuel injection valve, or built up, which corresponds to the opening position of the fuel injection valve, or the noninjection position. According to the invention, the throttle conduit segment is embodied substantially in the form of a variable aperture. Because the throttle conduit segment is embodied as a variable throttle aperture, an intentional influence is exerted on the development of the flow in the throttle conduit cross section, by making the actual throttle conduit segment so short that in its action it is like a variable aperture, in which when there is a flow to it, the conduit portions preceding the variable aperture and the fuel injection valves downstream of the variable aperture are substantially untouched by the flow at their conduit walls. This creates less flow resistance, on the one hand, so that the control behavior of the fuel injection valve can be improved considerably. On the other, because the throttle conduit cross section is embodied as a variable aperture, the complicated and expensive superfine surface machining in the otherwise relatively narrow flow conduit is dispensed with. As a result, the production costs can be reduced. Moreover, this reduces the influence of the precision of production on the flow behavior in such a throttle, since the flow through the immediate region of the throttle is substantially independent of the form of the surface of the remainder of the conduit.
The throttle conduit segment preferably has an l/d ratio such that cavitation occurs intentionally. Via a special geometric embodiment, especially a short length, it is also attained in this exemplary embodiment that the flow, when there is a flow through the actual throttle conduit segment, substantially does not press against its conduit walls, thus reducing the flow losses.
Preferably, the throttle conduit segment has an l/d ratio in the range from 0.1 to xe2x89xa62, in particular 1.0 to 1.5. In this connection, it can be noted that in particular in accordance with theoretical tests with l/d ratios in the range from 2 to 3, no cavitation occurs. In a practical sense, however, the cavitation does occur but is reduced. To preserve the original effect, namely to employ cavitation intentionally so that the flow will not press against the conduit wall, the length of the throttle conduit segment in proportion to its diameter is reduced considerably, thus achieving greatly reduced l/d ratios. Especially at very low l/d ratios, the throttle conduit segment assumes the form of a variable throttle aperture.
To exert a further purposeful influence on flow conditions inside the throttle, in a further preferred feature of the invention, the throttle conduit segment is embodied in a transition region with cross-sectional enlargement in the direction toward the control chamber in rounded fashion, in particular HE-rounded. Because of the rounded embodiment of the transitional region, the constriction of the fuel stream is reduced, which further reduces flow losses in the flow through the actual throttle conduit cross section or variable throttle aperture.
Preferably, the throttle has a first throttle conduit, which is closable by means of a closing element of the magnet valve, and a second throttle conduit, which discharges into the control chamber, and the variable aperture-like throttle conduit segment or variable throttle aperture is disposed between the first throttle conduit and the second throttle conduit in an approximately axial direction to one another. The diameters of the throttle conduits are embodied such that flow losses are kept relatively low, and the variable aperture like throttle conduit segment between the two throttle conduits is made so short that it takes on the form of a variable throttle aperture.